The present invention claims priority from European Patent application No. 01300479.1 filed Jan. 19, 2001 and herein incorporated by reference.
1. Field of the Invention
The present invention relates to lithographic projection apparatus and more particularly to lithographic projection apparatus including a vibration isolation system.
2. Description of the Related Art
Lithographic projection apparatus in accordance with the present invention generally include a radiation system for supplying a projection beam of radiation, a support structure for supporting patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern, a substrate table for holding a substrate, and a projection system for projecting the patterned beam onto a target portion of the substrate.
The term xe2x80x9cpatterning structurexe2x80x9d should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d has also been used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such pattering structure include:
A mask table for holding a mask. The concept of a mask is well known in lithography, and its includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. The mask table ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, which are incorporated herein by reference.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask table and mask; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallisation, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference
In a lithographic projection apparatus, it is necessary to control the relative position of the substrate table and the patterning structure with respect to the projection optics in the projection system to a very high degree of accuracy. Transient inaccuracies in this relative position, which may be caused by vibrations, may be detrimental to this accuracy. Lens vibrations may, for example, be caused by floor vibrations, indirect scanning forces (in the case of step-and-scan devices), noise in vibration isolation systems (originating in pneumatic suspension devices in the apparatus) or acoustic noise, among other things.
According to U.S. Pat. Nos. 5,953,105, 6,038,013, 5,187,519, EP 1 041 607 and GB 2 299 867 the above problem may be alleviated by mounting the projection system upon a main plate that is suspended from the rest of the apparatus with a vibration isolation system. The isolation vibration system isolates the main plate and the projection system from vibrations in the rest of the apparatus so that vibrations in the projection optics of the projection system are decreased.
A lithographic projection apparatus may require the positional error of the substrate table (and/or mask table) relative to the lens to be of the order of 2 nm or less. In addition, practical considerations in servo system design can demand that the positional stability of the lens be within tolerances of the order of 1 nm. The inventors have observed that positional errors of this magnitude may be produced by disturbance forces of the order of as little as 1N (acting on a machine that may have a mass of several hundred to several thousand kg). Typically, lithographic projection systems are particularly sensitive to vibrations with low frequencies in the range of 0 to 500 Hz. The desired degree of stability can therefore be very difficult to achieve.
It is an object of the present invention to provide an improved lithographic projection apparatus in which effective measures are taken to reduce the detrimental effect of lens vibrations.
This and other objects are achieved according to the invention in a lithography apparatus comprising:
a radiation system for providing a projection beam of radiation;
patterning structure, for patterning the projection beam according to a desired pattern;
a substrate table for holding a substrate; and
a silent world supported by a vibration isolation system and comprising a projection system for imaging the patterned beam onto a target portion of the substrate;
characterized by:
detection means for detecting relative movement between a first and a second portion of said silent world, and generating at least one movement signal representative thereof;
actuation means responsive to at least one control signal for exerting a force upon said silent world; and
control means responsive to said at least one movement signal for generating said at least one control signal, thereby to reduce relative movement between said first and said second portion.
The control provided by the present invention can substantially reduce the effect of vibrations (e.g. in the main frame or base plate of the device) on the relative positions of the lens and tables. This control can be specifically tuned to provide maximum compensation within particular frequency bands, e.g. around the eigenfrequency of the lens.
The present invention may be implemented using lens supports machined from single blocks, each block comprising said detection and actuator means. Co-location of detection and actuation means, such as to form a xe2x80x9csetxe2x80x9d (or several sets of detection and actuation means), enables the use of a control algorithm which is relatively simple compared to a control algorithm needed when detection and actuation means are not paired in sets. The lens support blocks may be manufactured such that they are compliant in some directions but stiff in others.
According to a further aspect of the invention there is provided a device manufacturing method using a lithographic projection apparatus comprising the steps of:
providing a substrate that is at least partially covered by a layer of radiation-sensitive material;
providing a projection beam of radiation using said radiation system;
using said patterning structure to endow the projection beam with a pattern in its cross-section;
projecting the patterned beam of radiation onto a target area of the layer of radiation-sensitive material using a projecting system provided to a silent world,
characterized by the step of
detecting relative movement between at least a first and a second portion of said silent world, and generating at least one movement signal representative thereof;
employing actuation means responsive to at least one control signal for exerting a force upon said silent world; and
employing control means responsive to said at least one movement signal for generating said at least one control signal, thereby to reduce relative movement between said first and said second portion of said silent world.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.
In the present document, the terms illumination radiation and illumination beam are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV, as well as particle beams, such as ion beams or electron beams.